Flat plate solar collectors are generally the most economical, simple and practical means of converting solar energy into heated water or other fluid. The simplest types of flat plate collector systems are those which operate on the so-called thermosiphon, wherein the bottom of the hot water storage tank is positioned vertically higher than the solar collector causing an automatic circulation of hot water from the collector up to the storage tank. Naturally, at night when the solar collector becomes quite cool because of its radiant properties, the cool water will remain in the collector and not be circulated into the hot water tank due to the elevational positioning of the two units. Proper functioning of the thermosiphon flat plate collector system requires positioning of the hot water storage tank above the collector, and thus an unsuitably large unit is required, or two separate units are required with suitable interconnecting plumbing that create greater expense for materials and installation than if the collector and storage tank could be efficiently packaged as a single unit.
Considerable effort and research has gone into the creation of a unit which will thermosiphon efficiently in the proper direction with the hot water storage tank positioned behind the collector panel. The main area addressed in the research was that of efficiently preventing reverse-siphoning of the unit at night so the hot water stored in the tank from the unit's daytime operation wouldn't be cooled. In essence, because the hot water tank is no longer elevated above the collector, in the evening and during other cold weather periods cool water in the collector will cause reverse-siphoning of the system, gradually filling the hot water tank up from the bottom with water which has been drawn from the top of the hot water tank, cooled in the collector which is now radiating heat, and then introduced as cooled water in the bottom of the hot water tank. This reversing-siphoning is precisely the system one would want if utilizing the stored water for space cooling in the daytime and operating the system at night. However, this is not ordinarily the case.
The reverse-siphoning difficulty cannot be resolved by putting in an ordinary check valve, because the flow forces are so minute that forward siphoning cannot be inhibited even to the extent of overcoming a check valve flap. Thus, research has turned toward the development of an effective, "thermic diode", which is simply a suitable check valve device that will stop or inhibit reverse flow and not substantially hinder forward flow.
A thoroughly effective and simple for achieving this and one with which the instant inventor has knowledge, utilizes a chamber having an immiscible fluid floated on top of the working fluid of the system. A stub pipe extends to approximately the inter-liquid surface to provide for the flow of water in the forward direction out of the pipe. It requires a minute amount of energy for forward flow because the oil or other fluid floating atop the working fluid is of a very slightly different specific gravity and is very easily raised a small fraction of an inch for the introduction of working fluid past the floating oil. However, if the system attempts to reverse-flow, it draws an increasingly long column of oil vertically down into the stub pipe, causing an increasingly negative pressure head in this pipe, eventually causing a neutral net pressure head in the entire system which stops the flow. This might work, but only if the descending pipe is insulated.
Although the floating oil technique is effective, it is naturally somewhat delicate in its parameters and is not necessarily adapted to being slung around by workmen and installed at haphazard angles on roofs of varying slopes and, therefore, while providing a definite step forward in the art of integral flat plate collector systems, does not provide a fail-safe, rugged system that can withstand a considerable amount of abuse and installation at any angle and still function properly.